A base oil for a traction oil used in a traction type CVT (continuously variable transmission) for automobiles necessarily satisfies contradictory conditions, i.e., a high traction coefficient at high temperature and a low viscosity at low temperature, and at the same time, preferably has a flash point of 150.degree. C. or more from the standpoint of safety on practical use. However, synthetic naphthenic compounds having high traction coefficients at high temperature exhibit poor low temperature fluidity. Thus, the low temperature fluidity must be preferentially realized with indispensable slight decrease of the high temperature traction coefficient. In this case, the molecular weight of the compound must be lowered, and as a result, the flash point is decreased to cause a problem of lacking practical utility. That is, because synthetic naphthenic compounds having high traction coefficients at high temperature have higher viscosity than paraffinic compounds having the same molecular weight, the molecular weight of the naphthenic compounds must be low to satisfy the low temperature fluidity. However, the low molecular weight brings about high volatility of the base oil to lower the flash point. It is generally preferred that the flash point of a lubricating oil for an automobile is maintained at 150.degree. C. or more upon practical use. Therefore, there has been a demand that the flash point of the naphthenic synthetic lubricating oil used as a traction oil for an automobile is improved without decreasing the high temperature traction coefficient, so as to satisfy all the high temperature traction coefficient, the low temperature fluidity, and the flash point.
In order to solve the problem of the naphthenic synthetic lubricating oil, there has been conducted an attempt to add a poly-.alpha.-olefin (JP-A-1-149898 and JP-A-3-103495). In these techniques, however, since the viscosity of the poly-.alpha.-olefin is too high, the addition amount thereof becomes too large to improve the low temperature fluidity, and as a result, the high temperature traction coefficient is greatly decreased. Furthermore, because the poly-.alpha.-olefin has a volatility (boiling point) that is greatly different from the naphthenic synthetic lubricating oil, the content of the poly-.alpha.-olefin is increased by volatilization of the light component (synthetic naphthenic compound) to greatly lower the high temperature traction coefficient, which causes a problem of gross slip of a CVT.